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Brony User Group Cologne hosted by 16.5.2012

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Ruby #9 “reflection”

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“ is the noblest way to learn wisdom.” – ޸෉ࢠ Reflection

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Reflection bedeutet zur Laufzeit Informationen … über definierte Konstanten, Objekte und ihre Eigenschaften abfragen Reflection

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23.class # >> Fixnum "Hello World".class # >> String Pony.kind_of?(Meme) # >> true pinkie_pie.instance_of?(Pony) # >> true Pony.class # >> Class Array.include?(Enumerable) # >> true Array.included_modules # >> [Enumerable, Kernel] Array.ancestors # >> [Array, Enumerable, Object, Kernel, BasicObject]

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const_defined? instance_methods instance_variable_defined? … respond_to? method_defined? respond_to? method_defined?

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pinkie_pie.respond_to?(:speak) # >> true Pony.respond_to?(:speak) # >> false Pony.method_defined?(:speak) # >> true pinkie_pie.method_defined?(:speak) # >> NoMethodError

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class Pony; end class PinkyPie < Pony; end class Brony; end PinkyPie < Pony # >> true Pony <=> PinkyPie # >> -1 Pony <=> Pony # >> 0 PinkyPie <=> Pony # >> 1 Brony <=> Pony # >> nil :( Fun Fact

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Elemente des Programms zur Laufzeit … … finden (z.B. Objekte oder Konstanten) und … benutzen (z.B. Methoden aufrufen) Reflection ist aber auch …

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# Get the 'Pony' class by name clazz = Object.const_get("Pony") # Create an instance instance = clazz.new("Rainbow Dash") # Call the 'speak' method dynamically instance.send(:speak) # >> 10 seconds flat!

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Methoden dynamisch aufrufen? Wie viele Parameter müssen übergeben werden? (arity) Wie heißen die Parameter? (parameters) Welchem Objekt ist die Methode zugeordnet? (owner und receiver) Object#method(sym) → method

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def meth() end # Params: [] # Arity: 0

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def meth(a) end # Params: [[:req, :a]] # Arity: 1

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def meth(a=42) end # Params: [[:opt, :a]] # Arity: -1

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def meth(a, *b) end # Params: [[:req, :a], [:rest, :b]] # Arity: -2

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def meth(*a, b) end # Params: [[:rest, :a], [:req, :b]] # Arity: -2

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def meth(a, *b, &c) end # Params: [[:req, :a], [:rest, :b], [:block, :c]] # Arity: -2

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def meth(a, *b, c) end # Params: [[:req, :a], [:rest, :b], [:req, :c]] # Arity: -3

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arity() parameters() () (a) (a=42) (a, *b) (*a, b) (a, *b, &c) (a, *b, c) 0 [] 1 [[:req, :a]] -1 [[:opt, :a]] -2 [[:req, :a], [:rest, :b]] -2 [[:rest, :a], [:req, :b]] -2 [[:req, :a], [:rest, :b], [:block, :c]] -3 [[:req, :a], [:rest, :b], [:req, :c]] -n-1* *n: parameters.select {|p| p.first == :req}.count

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ObjectSpace

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ObjectSpace.count_objects # { # :TOTAL => 14716, # :FREE => 420, # :T_OBJECT => 10, # :T_CLASS => 481, # ... # } ObjectSpace.each_object(Pony) { |obj| puts obj } # >> #, # ObjectSpace.each_object(Meme) { |obj| puts obj } # >> Pony

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“Ein [vollständig] reflexives System kann seinen eigenen Zustand während der Laufzeit abfragen und verändern. Veränderungen wirken sich dabei zur direkt auf das System aus.” Brian C. Smith – Reflection and Semantics in Lisp (1983)

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Das Verändern von Elementen des Programms zur Laufzeit z.B. durch … … Hinzufügen … Ändern oder Entfernen Reflection beschreibt also auch noch …

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# Define new Class 'Foo' Object.const_set("Foo", Class.new) puts Foo.instance_methods(false) # >> [] # Add a method to that class Foo.send(:define_method, "bar") do puts "Hello World!" end Foo.new.bar # >> 'Hello World!'

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Beispiel

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Enumerationen Datentyp mit festem Wertebereich Wertebereich wird bei Deklaration festgelegt Wertebereich ist sortiert z.B.: Farben, Städte, Länder

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enum "Colors", %w(Red Blue Yellow) Colors.each { |color| puts color.name } # Red # Blue # Yellow # … Colors::Red.name # >> Colors::Red Want:

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Need: def enum(name, elements) enum_class = Class.new do def self.each(&block) @_elements.each(&block) end def name self.class.name end end elements = elements.map do |el| enum_class.const_set(el, Class.new(enum_class)) end enum_class.instance_variable_set("@_elements", elements) Object.const_set(name, enum_class) end

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Dinge löschen …

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pinkie_pie = Pony.new pinkie_pie.sing # >> "I'm a singing Pony!" class Discord def self.silence! Pony.send(:remove_method, :sing) end end Discord.silence! begin pinkie_pie.sing rescue => e puts "Ponies cannot sing anymore!" end # >> 'Ponies cannot sing anymore!'

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Typen zur Laufzeit ermitteln Konstanten, Objekte & Methoden dynamisch finden dynamisch aufrufen dynamisch manipulieren und löschen! Reflection ermöglicht Meta-Programming

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One More Thing …

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No content

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Controller, View-Variablen, ...?

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{{@title}}

Welcome to Ruby is Magic

Today we welcome you to episode #{{@episode[:number]}} - {{@episode[:title]}}

{{@episode[:description]}}

Template

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Controller class Controller < AbstractController def index @title = "Ruby is Magic" @episode = { number: 9, title: "Reflections", description: "We gonna show you the magic of reflections in Ruby!" } render :index end end

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“Magic” class AbstractController def render(template) view = View.new(view_assigns) rendered_view = view.render(template) Rack::Response.new([rendered_view], 200, {"Content-Type" => "text/html"}) end private def view_assigns assigns = {} instance_variables.each do |variable| assigns[variable] = instance_variable_get(variable) end return assigns end end

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“Magic” class View def initialize(assigns) assigns.each do |name, value| instance_variable_set(name.to_s, value) end end def render(template) TEMPLATES[template].gsub(/{{(.*?)}}/) do |m| instance_eval($1) end end end

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$ curl http://localhost:9292 Ruby is Magic

Welcome to Ruby is Magic

Today we welcome you to episode #9 - Reflections

In this brand new episode we gonna show you the magic of reflections in Ruby!

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Thanks! Q & A? Dirk Breuer / @railsbros_dirk Sebastian Cohnen / @tisba ? “My Little Pony” © Hasbro Studios and DHX Media Vancouver rubyismagic.de